Apparatus and method for steering a vehicle

ABSTRACT

A steering system for a vehicle, the steering system includes a rack-independent actuator. The rack-independent actuator has component parts isolated from undesirable loads by two universal joints that isolate mechanical components of the actuator from transient loads that may be encountered by the rack or rack housing.

This is a divisional of U.S. Ser. No. 09/920,181, filed Aug. 1, 2001 nowU.S. Pat. No. 6,488,115 which is related to U.S. patent application Ser.No. 09/664,850, filed Sep. 19, 2000, the contents of which areincorporated herein by reference thereto which is also related to U.S.patent application Ser. No. 09/650,869, filed Aug. 30, 2000, thecontents of which are incorporated herein by reference thereto which isalso related to U.S. patent application Ser. No. 09/663,549, filed Sep.18, 2000, the contents of which are incorporated herein by referencethereto.

TECHNICAL FIELD

This invention relates generally to an apparatus and method for steeringa vehicle, and more specifically to a rack-independent actuator.

BACKGROUND

Many current steering system designs have replaced the hydraulic powersteering pump with electrically assisted systems based on fuel economy,modularity, engine independence, and environmental issues.

With electrically actuated or electrically assisted steering systemsthere is a significant servo mechanism design challenge associated withthe need to maintain proper kinematical constraint, while at the sametime, providing reasonable insulation from the drawbacks of tolerancestack up which may produce system lock up.

Although a successful servo mechanism design may appear to be acombination of basic “catalogue” mechanisms (e.g. ball-screw, gears,belts, various joints, etc.), the way these are used in combinationrepresents an unmistakably cardinal feature of this art.

The current state of engineering meets these concerns by anticipatingthe stresses likely to be encountered by designing heavy-dutycomponents. Needless to say, these designs are expensive to manufacture,have excessive performance challenges because of the increased inertiaand friction, and add to the overall weight of the vehicle.

SUMMARY OF THE INVENTION

The system is powered by a rotary type electric motor. The motor hasspeed reducers and rotary-to-linear actuators to achieve feasible sizeand linear actuation. The actuation unit is decoupled from thedirectionally unwanted loads by providing universal joints (or anequivalent degree of freedoms) at either end. One universal joint ismounted to the housing that holds the motor rotary-to-rotary speedreducer and the movable shaft of the linear-to-rotary actuator, and theother is mounted to a member that is linearly moved by thelinear-to-rotary actuator.

The use of universal joints (or gimbals), which provides kinematicaldegrees of freedom to prevent non-axial loads, also prevents bendingmoments on the rotary-to-linear actuator. In particular, such loads mayresult from the misalignment of the shafts and/or non-axial loading fromother components. This situation may produce undesirable friction andhigh stresses resulting in loss of efficiency and/or undesirablesteering feel. By avoiding the non-axial loads, the mechanizationbecomes feasible for all types of linear-to-rotary mechanizations, whichtoday are limited to very special ball-screws.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a steering system for a vehicle;

FIG. 2 is an illustration of a portion of the steering system in FIG. 1;

FIG. 3 is a perspective view of a rack-independent actuator constructedin accordance with an exemplary embodiment of the present invention;

FIG. 4 is a cross-sectional view of a rack-independent actuatorconstructed in accordance with an exemplary embodiment of the presentinvention;

FIG. 5 is another perspective view of a rack-independent actuator;

FIG. 6 is an end view of a rack-independent actuator constructed inaccordance with an exemplary embodiment of the present invention;

FIG. 7 is a top plan view of a rack-independent actuator constructed inaccordance with an exemplary embodiment of the present invention;

FIGS. 8 and 9 are perspective views of a rack-independent actuatorillustrating the universal joints in an exploded view;

FIG. 10 is an end perspective view of the rack-independent constructedin accordance with an exemplary embodiment of the present invention;

FIG. 11 is a partial cross sectional perspective view of arack-independent actuator constructed in accordance with an exemplaryembodiment of the present invention;

FIG. 12 is a partial cross sectional perspective view of a universaljoint of a rack-independent actuator constructed in accordance with anexemplary embodiment of the present invention;

FIG. 13 is a partial cross sectional perspective view of arackindependent actuator constructed in accordance with an exemplaryembodiment of the present invention;

FIG. 14 is a partial cross sectional perspective view of a universaljoint of a rack-independent actuator constructed in accordance with anexemplary embodiment of the present invention;

FIG. 15 is a block diagram of a rack-independent actuator system;

FIG. 16 a diagrammatic view of a steer by wire system; and

FIG. 17 is a diagrammatic view of a steer by wire system withindependent actuators for each steerable wheel of a vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The independent actuator system of an exemplary embodiment of thepresent invention employs the judicious use of universal joints,(gimbal) expansion joints, or other equivalents to achieve freedom fromlock-up as well as compensation for reasonable tolerance stack-uperrors, which must be designed around current steering system designs.

A benefit of the Electric Power Steering and Steer-by-Wire system is theenhanced comfort to the driver of a vehicle equipped with this system.The driver of such a vehicle would experience improved handling overless-than-smooth terrains e.g., potholes, graded surfaces, etc.

Less-than-smooth terrain increases the loads and deflections encounteredby the steering system. Thus, any bumps experienced by the vehicle mayincrease the wear and tear to the steering system, thus shortening andreducing its effective life.

Referring now to FIGS. 1 and 2, a steering system 10 for use in avehicle (not shown) is illustrated. Steering system 10 allows theoperator of the vehicle to control the direction of the vehicle throughthe manipulation of steering system 10.

A steering column 14 provides mechanical manipulation of the vehicle'swheels in order to control the direction of the vehicle. Steering column14 includes a hand wheel 16. Hand wheel 16 is positioned so that a usercan apply a rotational force to steering column 14. An upper steeringcolumn shaft 18 is secured to hand wheel 16 at one end and columnuniversal joint 20 at the other. Column universal joint 20 couples uppersteering column shaft 18 to a lower steering column shaft 22. Lowersteering column shaft 22 is secured to column universal joint 20 at oneend and a gear housing 24 at the other. Gear housing 24 includes apinion gear 26 (FIG. 2). Pinion gear 26 of gear housing 24 is positionedto make contact with a matching toothed portion 28 of a rack assembly30. Pinion gear 26 has helical teeth that are meshingly engaged withstraight-cut teeth of matching toothed portion 28.

The pinion gear, in combination with the straight-cut gear teeth of therack, form a rack and pinion gear set. The rack 45 is coupled to thevehicle's steerable wheels with steering linkage in a known manner.

Tie rods (only one shown) 32 are secured to rack assembly 30 at one endand knuckles 34 (only one shown) at the other.

As a rotational force is applied to steering column 14, through themanipulation of hand wheel 16 or other applied force, the pinion gear ofgear housing 24 is accordingly rotated. The movement of the pinion gearcauses the movement of rack assembly 30 in the direction of arrows 36,which in turn manipulates tie rods 32 and knuckles 34 in order toreposition wheels 36 (only one shown) of the motor vehicle. Accordingly,when the steering wheel 16 is turned, rack 45 and pinion gear 26 convertthe rotary motion of the steering wheel 16 into the linear motion ofrack 45.

In order to assist the user-applied force to the steering system, anelectric motor 38 is energized to provide power assist to the movementof rack 45, aiding in the steering of the vehicle by the vehicleoperator.

Electric motor 38 provides a torque force to a motor pulley 40 via motorshaft 42. The rotation force of motor pulley 40 is transferred to a belt44. There are retaining walls on either one of the pulleys 40 and/orball-screw pulley 62 to help prevent belt 44 from slipping completelyoff. Alternatively, motor pulley 40 can be configured to have noretaining walls. In yet another alternative, belt 44 is replaced by achain or gear system or any rotary to rotary drives that provides arotational force to the screw 64-of the ball-screw mechanism.

Accordingly, and as a torque force is applied to the belt 44, therotational force is converted into a linear force via therotary-to-linear actuator (ball-screw assembly 66), and rack 45 is movedin one of the directions of arrows 36. Of course, the direction ofmovement of rack assembly 30 corresponds to the rotational direction ofmotor pulley 40. Belt 44 has an outer surface and an inner engagementsurface. The configuration belt 44 and the position of electric motor 38allows inner engagement surface of belt 44 to wrap around and engageboth the motor pulley 40 and ball-screw pulley 62, that are fixed to therotary portion of a ball-screw 66 (rotary to linear actuator) mechanism.

Electric motor 38 is actuated by a controller 52 that receives inputsfrom a torque sensor 54 and a rotational position sensor 56. Sensor 56provides a steer angle signal to controller 52.

In addition, and as the motor shaft 42 of electric motor 38 turns, themotor shaft position signals of each phase are generated within electricmotor 38 and are inputted into controller 52 through a bus 58.

Controller 52 also receives an input in the form of a vehicle speedsignal. Accordingly, and in response to the following inputs: vehiclevelocity input; operator torque input (sensor 54); steering pinion gearangle (sensor 56); and motor shaft 42 position signals (bus 58),controller 52 determines the desired electric motor's current phases andprovides such currents through a bus 60.

Motor pulley 40 is rotated by motor shaft 42 of electric motor 38. Asecond pulley 62 is fixedly secured to the ball-screw 64 (or the rotarypart of a rotary to linear actuator) of a ball-screw assembly 66. Theball-screw assembly 66 converts the rotary force of belt 44 into thelinear movement of a ball nut 68.

Motor pulley 40 and ball-screw pulley 62 may be constructed out of alightweight material such as aluminum or composites. This allows theoverall mass and inertia of steering system to be reduced in order toimprove manufacturing costs and performance, as well as vehicle fuelefficiency.

FIGS. 1 and 2 illustrate a power assist steering system which includes amechanical connection between (rack and pinion) hand wheel 16 and rackassembly 30.

Alternatively, and in applications in which a “steer-by-wire system” isemployed, there is no direct mechanical connection between hand wheel 16and rack assembly 30. In this application, the driver's rotationalmovement of the hand wheel 16 (and/or signal from an equivalent drivercontrol device such as a joystick, pedal(s) and other mechanism formanipulation by the vehicle operator) is input into the controller 52while electric motor 38 provides the necessary force to manipulate rackassembly 30.

Referring now to FIGS. 3-14, a rack-independent actuator 70 isillustrated. In accordance with an exemplary embodiment,rack-independent actuator 70 provides the necessary force to effect thelinear movement of a rack 45 coupled to the steerable wheels of avehicle. Rack-independent actuator 70 performs the functions of rotatingthe steerable wheels of a vehicle in response to an input such as drivermanipulation of a steering wheel. In addition, and while performing thisfunction the rack independent actuator 70 isolates its reductionmechanisms and/or conversion mechanisms necessary to effect the rotationof the steerable wheels from transient and non-axial (to the rack) loadsby a pair of universal joints 72 and 74.

Rack-independent actuator 70 is also contemplated for use with a powerassist steering system (FIGS. 1 and 2) and/or a “steer-by-wire system”(FIGS. 16 and 17) and/or rear wheel steering and/or four-wheel steering.

FIGS. 8 and 9 illustrate universal joints 72 and 74 in an exploded viewin order to illustrate the component parts of the same.

Universal joint 72 secures a housing 75 to a mounting member 76 of rackassembly 30. Universal joint 72 contains two sets of hinge pins, orpivots 78 and 80, the axis of each set being perpendicular to the other.Each set of pins is connected to the other by a central gimbal ring 82.

As an alternative, universal joints 72 and 74 may be replaced by acompliant member which allows similar degrees of freedom for the rangeof motion necessary to isolate the reduction mechanisms from transientand non-axial (to the rack) loads. For example, gimbal ring 82 isreplaced by a rubber ring which is inserted into mounting member 76while also covering a portion of housing 75. The rubber ring iscompressible and thus capable of providing kinematic freedom. Similarly,gimbal ring 92 may be replaced by a compliant rubber ring.

In yet another alternative, rack independent actuator may be constructedwith a universal joint and a rubber compliant member. For example,universal joints 72 and a rubber compliant member replacing universaljoint 74 or vice versa.

In an exemplary embodiment, pins 78 and 80 are pressed at theirrespective openings in gimbal ring 82. This allows the rotationalmovement of gimbal ring 82 while also providing a means for securing thesame. Alternatively, pins 78 and 80 slip in openings in housing 75 andmounting member 76.

Alternatively, pins 78 and 80 and their respective openings in gimbalring 82, housing 75 and mounting member 76 are configured to provide amovable means of securing the same.

Pins 78 movably connect gimbal ring 82 to housing 75. In an exemplaryembodiment, housing 75 is configured to have an elongated cylindricalshape allowing a portion of housing 75 to be inserted within an inneropening of gimbal ring 82. Thus, pins 78 allow gimbal ring to be movablysecured to housing 75.

In addition, pins 80 movably connect gimbal ring 82 to mounting member76. Mounting member 76 is fixedly secured to an outer housing 77 of rackassembly 30. In an exemplary embodiment, mounting member 76 defines aninner opening 88 sufficiently large enough to pass over gimbal ring 82.

Accordingly, gimbal ring 82 is movably secured to housing 75, andhousing 75 is sufficiently long enough to position gimbal ring 82 withinopening 88 of securement member 76, thus gimbal ring 82 connects housing75 and securement member 76 by pins 78 and 80. Pins 78 pass throughopenings 73 in securement member 76 and movably secured gimbal ring 82to securement member 76, while pins 80 movably secure gimbal ring 82 tohousing 75 by engaging openings 81 in housing 75. In an exemplaryembodiment, pins 78 and 80 are positioned at right angles with respectto each other. Of course, the angular positioning of pins 78 and 80 mayvary as long as the intended effect of isolating portions of the rackindependent actuator from unwanted loads is achieved.

For example, pins 80 prevent a load from being transferred in-betweenmounting member 76 and gimbal ring 82 in a first direction while pins 78prevent a load from being transferred in-between housing 75 and gimbalring 82 in a second direction. The first and second directions beingdifferent from each other.

As an alternative, and in order to prevent a load from being transferredto gimbal ring 82 and/or gimbal ring 92 the pins which secure the gimbalrings are covered with plastic and/or rubber to further enhance theisolation of the mechanism from unwanted loads.

Rack-independent actuator 70 has an electric motor assembly 90. Electricmotor assembly 90 includes electric motor 38, rotatable shaft 42, andmotor pulley 40 that is fixedly secured to motor shaft 42. As pulley 40is rotated by motor shaft 42, belt 44 engages with pulley 40 as well aspulley 62. Since pulley 62 is fixedly secured to screw 64 of theball-screw mechanism, the rotational movement of pulley 62 causes screw64 of the ball-screw mechanism to rotate. Accordingly, motor 38, belt44, pulleys 40 and 62 provide a rotary to rotary conversion, which isdetermined by the dimensions of pulley 40 and 62 with respect to eachother (e.g. gear ratio).

As an alternative and in accordance with the present invention it iscontemplated that other mechanisms and means for rotary to rotaryconversion may be employed with the present invention. For example,pulleys 40 and 62 and belt 44 can be replaced by a direct mechanicallinkage such as a gear train rotary to rotary drive or equivalentthereof.

One end of screw 64 of the ball-screw mechanism is mounted for rotationwithin a plurality of bearings 65 located within housing 75 proximate topulley 62. A pre-load nut adjuster or locking nut 67 screws onto thescrew 64 of the ball-screw mechanism adjacent to bearings 65, once inposition locking nut is secured to screw 64 of the ball-screw mechanismthrough the use of a plurality of locking screws 63 which when rotatedlock locking nut 67 onto screw 64 of the ball-screw mechanism. Thus,bearings 65 are positioned between locking nut 67 and pulley 62 allowingfor the rotational movement of screw 64 of the ball-screw mechanism. Theother end of screw 64 of the ball-screw mechanism is rotatably supportedby ball-screw nut 68 of ball-screw mechanism 66. Accordingly, therotational movement of screw 64 of the ball-screw mechanism by motor 38is isolated at one end by universal joint 72.

A portion of screw 64 of the ball-screw mechanism passes throughball-screw nut 68, and the respective surfaces of screw 64 of theball-screw mechanism and ball-screw nut 68 are configured to effect thelinear movement of ball-screw nut 68 as screw 64 of the ball-screwmechanism is rotated. In an exemplary embodiment, a plurality of balls69 are received within a pair of threaded or grooved surfaces 71positioned on the inner surface of ball-screw nut 68 and the outersurface of screw 64 of the ball-screw mechanism. The interface of screw64 of the ball-screw mechanism and ball-screw nut 68 of ball-screwmechanism 66 are constructed in a known manner.

Accordingly, and as screw 64 of the ball-screw mechanism is rotated bythe rotational movement of pulley 62 by motor 38, the rotationalmovement of screw 64 of the ball-screw mechanism is converted intolinear movement of ball-screw nut 68. It is here that rotary to linearconversion occurs. As an alternative, other means for rotary to linearconversion are contemplated for use with the present invention.

The interface between ball-screw nut 68 and rack 45 is isolated byuniversal joint 74. Ball-screw nut 68 is secured to a gimbal ring 92 ofuniversal joint 74. Similarly to universal joint 72, universal joint 74contains two sets of hinge pins or pivots 94 and 96, the axis of eachset being perpendicular to the other. Each set of pins is connected tothe other by central gimbal ring 92.

In an exemplary embodiment, pins 94 and 96 are pressed in theirrespective openings in gimbal ring 92. This allows the rotationalmovement of gimbal ring 92 while also providing a means for securing thesame.

Alternatively, pins 94 and 96 and their respective openings in gimbalring 92, ball-screw nut 68 and housing member 100 are configured toprovide a movable means of securing the same.

Pins 94 movably connect gimbal ring 92 to ball-screw nut 68 allowing formovement in a first direction. In an exemplary embodiment, gimbal ring92 is configured to have a cylindrical shape slightly larger thanball-screw nut 68, allowing a portion of ball-screw nut 68 to beinserted within gimbal ring 92. Pins 94 are received within a pair ofpin openings 98 in the ball-screw nut 68. It is noted that universaljoint 74 and ball-screw nut 68 are shown in FIGS. 8 and 9 in an explodedmanner so as to illustrate the attachment of universal joints 72 and 74.

Pins 96 movably connect gimbal ring 92 to a housing member 100 allowingfor movement in second direction, the second directional plane beingorthogonal to the first directional plane. Pins 96 pass through a pairof apertures 102 in housing 100, thus movably connecting gimbal ring 92to housing 100.

The gimbal mechanisms or in particular universal joints 72 and 74provide the necessary kinematic degrees of freedom to prevent non-axialloads and for turning or bending moments on the ball-screw nut or screw,such as those that would result from misalignment of the shafts, fromproducing undesirable friction and the resultant loss of efficiency onthe rotary to linear motion conversion mechanism.

In so doing, the torque output and power consumption requirements of themechanism used to turn the ball-screw such as the electric motor isreduced. This allows the electric motor to be reduced in size as well asthe components of the rotary to linear actuator. This is particularlyuseful for applications such as vehicular electric steering actuators,where the dynamic loads experienced by the vehicle and the requirementsplaced on the mechanism can significantly impact the motor and actuatormechanism requirements. The reduction in power consumption of the motorand the weight reductions associated with a smaller electric motor andmechanism represent desirable to design parameters.

Referring now in particular to FIG. 4, housing 100 is fixedly secured torack 45 through a plurality of bolts 104 which pass throughcomplementary bolt openings 106 in rack 45 and housing 100. Accordingly,and as a rotational force is applied to screw 64 of the ball-screwmechanism, ball-screw assembly 66 converts the rotary movement of screw64 of the ball-screw mechanism into the linear movement of ball-screwnut 68. Ball-screw nut 68 is connected to rack 45 through a universaljoint 74, which is connected to ball-screw nut 68 at one end and housing100 at the other. Housing 100 is fixedly secured to rack 45 andaccordingly, as ball-screw nut 68 moves in the direction indicated byarrows 36, a similar movement of rack 45 is produced.

Housing member 100 is configured to have a mounting portion 101 which isconfigured to be received within opening 108. Mounting portion 101 isconfigured to be slidably received within opening 108 and contains theapertures into which bolts 104 are received.

Universal joints 72 and 74 isolate electric motor assembly 90 andball-screw pulley 62 from transient non-axial loads, which may damage ormisalign pulleys 40 and 62. Moreover, universal joints 72 and 74 isolatethe system from undesirable loads or stack buildup which may be theresult of misalignment of a component part such as rack 45, ball-screw64 and/or any other component part which may produce an undesirable loador stack buildup.

The rack-independent actuator also allows the two pulleys on the beltand pulley mechanism to be mounted to the same housing and to eliminateall force components that could alter their parallelism.

Moreover, the rack-independent actuator of an exemplary embodiment nolonger requires the motor shaft of motor 38 or the screw 64 of theball-screw mechanism to be parallel to rack 45, as motor assembly 90 andscrew 64 of the ball-screw mechanism are isolated from rack 45 throughthe use of universal joints 72 and 74. Thus, any misalignment of screw64 of the ball-screw mechanism with regard to rack 45 is accommodatedfor by universal joints 72 and 74. Accordingly, motor shaft 42 need onlybe parallel to screw 64 of the ball-screw mechanism, or alternatively,pulleys 40 and 62 need only be parallel to each other. Accordingly, andsince they are mounted to the same housing, this is easily achieved andmaintained. Moreover, any loads which may cause misalignment areisolated from the motor assembly through the use of universal joints 72and 74.

Also, pulleys 40 and 62 may be configured with or without retainingwalls because, as stated above, belt 44 is isolated from transientforces, thus reducing belt/pulley production costs, since the belt andpulley system does not have to be designed to withstand large forces.

Referring back now to FIGS. 4, 8, 9 and 11-14, outer housing 77 of rackassembly 30 is configured to have an elongated opening 108. In order toprevent the rotational motion of the rack 45, an anti-rotation device110 is secured to rack 45 (FIG. 4) that moves within the confinement ofthe elongated opening 108.

In an exemplary embodiment, anti-rotation device 110 is a plug 112fixedly secured within an opening 114 of rack 45. Plug 112 has an uppermember depending outwardly from rack 45, and is sized and configured topass along in elongated opening 108. In addition, and in order to reduceany frictional buildup between plug 112 and the elongated opening 108, aplurality of bearings 116 are positioned around the periphery ofanti-rotation device 110. Accordingly, anti-rotation device 110 preventsrotational movement of rack 45 while allowing linear movement of thesame.

Rack assembly 30 is also configured to have a pair of mounting members118. Mounting members 118 are configured to secure rack-independentactuator 70 to a vehicle frame (not shown).

In addition, and referring now to FIG. 4, housing 77 of rack assembly 30has a pair of apertures 120. Apertures 120 are positioned to allow atool such as a screwdriver or other type of tool to be inserted intoopenings 120 in order to facilitate the securement of bolts 104 tohousing 100 and rack 45.

The steering system is equipped with several sensors that relayinformation to the electric motor 38 by way of a controller 52 (FIG. 1).Controller 52 will track the position and force upon rack 45 at alltimes by means of a pair of force sensors 122. Force sensors 122 provideinput into controller 52 corresponding to the amount of force includedat the ends of rack 45.

A pair of absolute position sensors 124 and a high-resolution sensor 126also provide input into controller 52 in the form of a rack positionlocation. For example, an on-center position sensor may compriseHall-Effect devices, which are mounted within rack-independent actuator70. It may be understood that the sensors and controller 52 comprise acalibration means for maintaining the values of the steering positionsignals that correspond with the actual steering positions.

Rack 45 has a center position in which the steerable wheels of a vehicleare directed straight ahead relative to the vehicle. In an exemplaryembodiment, rack-independent actuator 70 will provide a return torquethat assists in returning the steering system to a center position.

In this system, the return torque is generated by electric motor 38, anda return torque component of the total desired torque signal isgenerated in controller 52 based upon the input received from sensors122, 124, and 126. Thus, an accurate signal of the steering position isderived from absolute position sensor 124.

In order to express the full range of steering angles as the output ofabsolute position sensor changes, the apparatus utilizes an algorithm incontroller 52. The algorithm may be embodied in a programmed digitalcomputer or a custom digital processor (not shown).

Referring now to FIG. 15, a block diagram illustrates the use of theuniversal joints and the unit interaction between various components ofthe rack-independent actuator system.

Block 130 represents the electric motor. Block 130 interfaces with block132 that represents the rotary-to-rotary assembly of therack-independent actuator system. Block 130 also interfaces with thehousing of the ball-screw indicated at block 134. Block 132 interfaceswith a block 136 that represents a rotary-to-linear assembly. Block 136interfaces with a block 138 that represents the bearings of theball-screw, and block 138 interfaces with the ball-screw housing. Block140 represents a high-resolution sensor that interfaces with the housing(block 134) and the rotary to linear assembly (block 136).

Block 142 represents an interface between the rotary-to-linear assemblyand the housing of the rack assembly.

Block 144 represents the housing of the rack assembly. Block 146represents an absolute position sensor which interfaces with box 136 andbox 144. Block 148 represents a tie rod and force sensor whichinterfaces with the housing of the rack assembly (block 144).

Block 150 represents the interface between housing 134 and the rackhousing 144. It is here at block 150 in which universal joint 72 orstationary universal joint 72 is inserted to isolate the motor and beltand pulley assembly from the housing of the rack assembly.

Block 142 represents the interface between the rotary-to-linear assemblyhousing and the rack assembly. It is here at block 142 in whichuniversal joint 74 or mobile universal joint 74 is inserted to isolatethe movement of the rack assembly from the ball-screw nut of theball-screw assembly.

This system accomplishes compensation through a series of sensors thatprovide feedback to several components. For instance, therotary-to-linear assembly at block 136 receives inputs from the absoluteposition sensors at block 146. In this embodiment, the absolute positionsensors are mounted to the ball-screw assembly. The absolute positionsensor at block 146 provides steer angle signals that are sent to thecontroller.

While an exemplary embodiment of the present invention has beendescribed with reference to a steering system for a vehicle, therotary-to-linear mechanism is not intended to be limited to suchapplications. It is contemplated that in accordance with the presentinvention, a rotary-to-linear conversion mechanism utilizing a pair ofuniversal joints for isolating the mechanism from misalignment and/oruneven loading can be applied to any application.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method for providing an actuation force to arack of a vehicle, comprising: selecting a first coupling mechanism anda second coupling mechanism from a group consisting of universal joints,gimbals, and cylindrically shaped compliant members; coupling anelectric motor to a rack housing with the first coupling mechanism;isolating non-axial loads from an the electric motor of a steeringsystem with the first coupling mechanism, said motor providing arotational force to a rotatable member of a rotary-to-linear conversiondevice; coupling the rotary-to-linear conversion device to the rack withthe second coupling mechanism; and, isolating non-axial loads from alinearly actuatable member of said rotary-to-linear conversion devicewith the second coupling mechanism, said linearly actuatable memberbeing coupled to a the rack of said steering system.
 2. A method forproviding an actuation force to a rack of a vehicle as claimed in claim1 wherein said isolating non-axial loads from said electric motorfurther comprises compliantly mounting said electric motor at saidsteering system and relative to said steering system with a compliantmember.
 3. A method for providing an actuation force to a rack of avehicle as claimed in claim 1 wherein said isolating non-axial loadsfrom said linearly actuatable member further comprises compliantlymounting said linearly actuatable member with a compliant member.
 4. Amethod for providing an actuation force to a rack of a vehicle asclaimed in claim 1 wherein said method further comprises: rotating saidelectric motor: rotating said first coupling mechanism about a portionof the rack housing; and rotating said rotatable member through saidfirst coupling mechanism.
 5. A method for providing an actuation forceto a rack of a vehicle as claimed in claim 1 wherein said method furthercomprises: rotating said electric motor; rotating said rotatable member;moving said linearly actuatable member; and moving said second couplingmechanism with said linearly actuatable member.
 6. The method of claim 1further comprising configuring at least one of said first and secondcoupling mechanisms to provide kinematical degrees of freedom to limiteffects of at least one of non-axial loads and bending moments actingthereon.
 7. The method of claim 1 further comprising at least partiallyrotating the first and second coupling mechanisms about an axis of arotatable member of the rotary-to-linear conversion device.
 8. Themethod of claim 1 further comprising coaxially aligning the first andsecond coupling mechanisms.
 9. A method for providing an actuation forceto a rack of a vehicle, comprising: movably mounting the rack within arack housing; providing a rotary-to-linear conversion device; coupling aportion of the rack to an electric motor with the rotary-to-linearconversion device, wherein the electric motor provides an actuatingforce to the rotary-to-linear conversion device, said actuating forcecausing the rack to move linearly along a first axis within the rackhousing; coupling the electric motor to the rack housing with a firstcoupling mechanism; coupling the rotary-to-linear conversion device tothe rack with a second coupling mechanism; and employing the first andsecond coupling mechanisms for isolating transient forces and forcesthat are not axially aligned to said first axis.
 10. The method of claim9 further comprising at least partially rotating the first and secondcoupling mechanisms about an axis of a rotatable member of therotary-to-linear conversion device.
 11. The method of claim 9 furthercomprising selecting the first and second coupling mechanisms from agroup consisting of universal joints, gimbals, and cylindrically shapedcompliant members.
 12. The method of claim 9 further comprising fixingthe first coupling mechanism with respect to linear movement relative tothe first axis and allowing the second coupling mechanism to movelinearly with a linearly actuatable member of the rotary-to-linearconversion device.
 13. The method of claim 9 further comprisingcoaxially aligning the first and second coupling mechanisms.
 14. Amethod for providing an actuation force to a rack of a vehicle,comprising: coupling an electric motor to a rack housing with the firstcoupling mechanism; isolating non-axial loads from the electric motor ofa steering system with the first coupling mechanism, said motorproviding a rotational force to a rotatable member of a rotary-to-linearconversion device; coupling the rotary-to-linear conversion device tothe rack with the second coupling mechanism; and, isolating non-axialloads from a linearly actuatable member of said rotary-to-linearconversion device with the second coupling mechanism, said linearlyactuatable member being coupled to the rack of said steering system;and, configuring at least one of said first and second couplingmechanisms to provide kinematical degrees of freedom to limit effects ofat least one of non-axial loads and bending moments acting thereon. 15.The method of claim 14 further comprising at least partially rotatingthe first and second coupling mechanisms about an axis of a rotatablemember of the rotary-to-linear conversion device.
 16. The method ofclaim 14 further comprising coaxially aligning the first and secondcoupling mechanisms.